\section{Introduction} 
\label{sec:introduction}

Experiments at the Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Laboratory have measured only 1/5 of the particles expected from Quantum Chromodynamics (QCD) at large transverse momentum (pT) [1]. QCD is the fundamental theory governing the strong interaction of quarks and gluons, the basic constituents of matter. QCD has been tested by all experiments to be the correct theory of nature. The measured suppression of high-pT particle yield is believed to be a consequence of energy loss (jet-quenching) [2] resulting from interactions of high-pT partons (quarks and gluons) with thermal partons of the medium created in heavy-ion collisions. In order for only 1/5 to survive, the medium parton density has to be very high, which can only exist in a deconfined state of quarks and gluons, or the quark-gluon plasma (QGP) [1]. The details of the energy loss mechanism is, however, not fully deciphered. 


Experiments can only measure hadrons, final products of partons that themselves cannot exist freely in vacuum. In order to gain insights, this research exploits A Multi-Phase Transport (AMPT) model [3]. The  AMPT provides the entire history of parton-parton interactions from the initial encounter of two colliding nuclei to the final-state partons which then are coalesced into hadrons (hadronization). In this study, we have so far concentrated only on final-state partons to avoid complications of hadronic scatterings. Although only parton-parton elastic scatterings are implemented in AMPT (no inelastic scatterings or gluon radiation), our study should provide valuable insights into the partonic energy loss mechanism and phenomenology.


The parton-parton collision history is saved to output data file by AMPT. We have written code to extract information from the data file to track the collision history of partons traversing the medium. We track each parton from initial state to final state before hadronization. We study the parton relative energy loss and azimuthal angle change. We study the correlation between these two quantities as well as the dependence of energy loss on the number of parton-parton collisions. 

